In cellular mobile radio systems, it is not unusual for a radio connection to be lost. The radio connection can be lost for a variety of reasons including fading when the mobile station is standing still, radio shadow from large buildings or tunnels, a handoff to a radio channel which can not support the new connection or when a base station breaks down or for some emergency must be switched off. As a result, a reestablishment routine is used in cellular systems to reconnect the lost connection.
FIG. 1 illustrates a prior art system for reestablishing a call in a cellular radio system. In this example, a standard telephone 109 communicates via a public switched telephone network PSTN (not shown) and a cellular radio communication system with a mobile radio station 104. The cellular radio communication system includes two radio base stations 101 and 102, which are both connected to the same mobile services switching center 103 by connections 107 and 108, respectively. In FIG. 1, the mobile station 104 is communicating with telephone 109 through base station 101. When the mobile station is moving toward the base station 102, it may become necessary to handoff the mobile station 104 from base station 101 to base station 102. However, if the mobile station 104 is disconnected from the base station 101 before receiving a handoff order to the new base station 102, the mobile station may initiate a reestablishment routine.
In this known reestablishment routine, the mobile station 104 scans the signal strengths of the surrounding base stations and selects the base station with the highest or next highest signal strength. The mobile station 104 then accesses the selected base station, base station 102 in this example, and requests to be reconnected. The selected base station reports the request for reconnection to the mobile services switching center 103. The mobile services switching center 103 then orders the selected base station to assign a traffic channel to the mobile station 104, and the mobile services switching center 103 then connects the mobile station to the remaining segment of the call which has been parked in the mobile services switching center 103.
In the European cellular mobile radio system (GSM), a call reestablishment procedure has been standardized which provides for call reestablishment within the same mobile services switching center. According to this standard, the mobile station accesses the best or next best base station and requests a reconnection which is provided if the accessed base station is connected to the same mobile services switching center as the original base station. However, this reestablishment routine is problematic in the sense that if the accessed base station is not connected to the same mobile services switching center, the reconnection can not be provided. For example, a subscriber with a hand held mobile station may enter or leave a building, thereby losing contact with its base station and enter an overlay network with another mobile services switching center. As a result, if the mobile station loses its connection with the base station, the call can not be reestablished. Furthermore, in non-cellular systems, an unwillful disconnection of a call associated with a wirebound terminal in a PABX or in the public switched telephone network PSTN, can only be reconnected by redialing. A faster method for reestablishing a call which does not require the user going on hook and redialing the number would therefore be welcome.